Self-supporting fiber optic cable

ABSTRACT

A self-supporting fiber optic cable includes a messenger section having at least one strength and anti-buckling member enclosed within a jacket and a carrier section enclosed within a jacket that is joined to the jacket of the messenger section by a web. In a preferred embodiment of the present invention, carrier section does not include strength members and the optical fibers are set with a high EFL. The greater EFL accommodates elongation of carrier section without transmission of stress to optical fibers. In addition, the preferably generally cylindrical internal surface of a tube or jacket curves the optical fibers creating EFL, for example, the fibers are guided by the internal surface in a helical path. Resistance to carrier section elongation and contraction can be controlled by varying the length of the web connecting the carrier and messenger sections.

RELATED APPLICATIONS

The present application is a Continuation of U.S. Ser. No. 09/579,555filed on May 26, 2000 now U.S. Pat. No. 6,546,175 which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to fiber optic cables and, moreparticularly, to self-supporting fiber optic cables.

BACKGROUND OF THE INVENTION

Fiber optic cables can be employed in a variety of applicationsincluding the transmission of voice, video, and data. Fiber optic cablescan be installed either in conduits that are disposed within the ground,or aerially by being strung between utility poles or the like.Self-supporting fiber optic cables can include both optical fibers aswell as strength members for providing mechanical support for theaerially installed cable.

Self-supporting fiber optic cables typically include a strength memberin the form of a messenger section supporting the fiber optic cable, anda carrier section that includes optical fibers, or optical fibers andelectrical conductors. One type of self-supporting fiber optic cable isa “figure 8 ” configuration. A “figure 8 ” self-supporting fiber opticcable includes a pair of cable sections connected by a web, wherein themessenger section forms one of the sections and the carrier sectionforms the other cable section. The web joining the messenger and carriersections typically extends continuously along the length of the cable.Plastic components of a cable have a significantly greater coefficientof thermal expansion than the glass fibers. With changes in temperature,the plastic components may elongate or contract. The strength members inthe messenger section help limit the amount of elongation andcontraction that can take place by virtue of the connecting web thatjoins the carrier section to the messenger section.

The carrier section can have an overlength with respect to the messengersection. See, for example, U.S. Pat. Nos. 4,662,712 and 4,883,671, thecontents of which are incorporated by reference herein.

In some field applications of figure 8 cables, the messenger and carriersections are separated from each other over substantial lengths. Forexample, it may be desired to install the carrier section alone down theside of a utility pole to a repeater or other device. To accommodatesuch uses, figure 8 cables have required respective strength members inboth the messenger carrier section.

SUMMARY OF THE INVENTION

One aspect of the present invention presents a fiber optic cable havinga messenger section, the messenger section having at least one strengthmember and a jacket surrounding the strength member. The cable includesa carrier section, the carrier section being free of strength membersand at least one optical fiber disposed therewithin, and a web connectsthe respective jackets of the messenger and the carrier sections.

Another aspect of the present invention presents a fiber optic cablehaving at least one optical fiber disposed within the jacket, theoptical fiber being at least partially disposed along a path and havingan EFL of about 0% to about 3.2% at unstressed room temperatureconditions.

Another aspect of the present invention presents the combination of acarrier section not including a strength member, and the optical fiberbeing at least partially disposed along a curved path defined by asurface having an inside diameter between about 3.0 mm to about 13.0 mm,the optical fiber having an EFL of about 0.6% to about 3.2%. Cablesaccording to the present invention can have connecting webs of varyinglengths to control elongation of the carrier section.

Another aspect of the present invention presents a fiber optic cablehaving a messenger section, the messenger section having at least onestrength member and a jacket surrounding the at least one strengthmember. A carrier section is connected to the messenger section by aseries of webs, the webs having a general axial length L, with windowsdefined between the webs, the webs having a general axial length W, theL:W ratio preferably being about 1:1 to about 50:1. Alternatively, theL:W ratio can be about 0.25:1 to about 0.95:1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a fiber optic cable according to onepreferred embodiment of the present invention taken along line 1—1 ofFIG. 2.

FIG. 1a is a cross-sectional view of a fiber optic cable having afilling compound disposed within the passageway of the jacket.

FIG. 2 is perspective view of the fiber optic cable of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, a fiber optic cable 10 according to apreferred embodiment of the present invention is illustrated. Fiberoptic cable 10 is a self-supporting cable that is particularly suitablefor aerial applications. Fiber optic cable 10 preferably has a “figure 8” configuration. In this regard, fiber optic cable 10 preferablyincludes messenger and carrier sections 12, 14 that define respectiveportions of the “figure 8”.

Messenger section 12 extends longitudinally and includes at least oneand, more preferably, a plurality of strength members 16. For example,the carrier section can include a single metallic strength member 16, ormore preferably a plurality of metallic wires, such as steel wires, thatare helically wound or otherwise twisted together. However, the strengthmembers can be non-metallic, including fiber-reinforced plastics such asglass reinforced plastic (GRP) or aramid reinforced plastic (ARP). Themessenger section also includes a jacket 18, preferably formed ofpolyethylene, that surrounds strength members 16. As shown in FIGS. 1and 2, the jacket has a preferably circular shape in lateralcross-section.

Carrier section 14 is connected to messenger section 12 as describedbelow and also extends in a longitudinal direction. Carrier section 14preferably does not include strength members. In the embodiment of FIGS.1 and 2, the carrier section includes a core tube 20. Preferably, coretube 20 comprises a single tube that is centrally disposed within thecarrier section. Core tube 20 is preferably formed of polypropylene,although the core tube can be formed of other materials.

Carrier section 14 also includes at least one and, more preferably, aplurality of optical fibers 22 disposed within core tube 20. Opticalfibers 22 preferably include a silica-based core that is operative totransmit light and is surrounded by silica-based cladding having a lowerindex of refraction than the core. A soft primary coating surrounds thecladding, and a relatively rigid secondary coating surrounds the primarycoating. Optical fibers 22 can be, for example, single-mode ormulti-mode optical fibers made commercially available by CorningIncorporated. Optical fibers 22 can be arranged within core tube 20 inbundles as depicted in FIG. 1; alternatively, optical fibers 22 can bearranged in loose, unbundled form, or in optical ribbon form 22 a.

In order to inhibit water migration in core tube 20, the carrier sectioncan also include a water-blocking substance disposed within the coretube and around the optical fibers. For example, the water-blockingsubstance can be a water-swellable element 23 having a super-absorbentpolymer, that can be incorporated into water-swellable yarns and/orwater-swellable tapes; alternatively or additionally, a filling compoundsuch as a hydrophobic grease or gel can fill core tube 20. If desired,carrier section 14 can include respective water-blocking tapessurrounding individual optical fibers, bundles, or optical ribbons,and/or a water-blocking tape can surround all optical fibers.

Carrier section 14 can also include a protective layer such as acorrugated armor tape (not shown) wrapped about core tube 20.Preferably, a waterblocking tape would be interposed between the armortape and the jacket. Such an armor tape can significantly enhance theruggedness of carrier section 14.

Carrier section 14 also includes a jacket 24 surrounding core tube 20.Preferably, the jacket of the carrier section is formed of the samematerial as the jacket of the messenger section, such as polyethylene.Although jacket 24 of carrier section 14 is illustrated as having agenerally circular shape in FIGS. 1 and 2, jacket 24 can have anoncircular shape if desired.

Although not necessary for the practice of the present invention,carrier section 14 preferably includes one or more rip cords 26 (FIG. 1)therewithin between the tube and the jacket. By pulling the rip cords inboth a radially outward and a longitudinal direction, the rip cords cansever core tube 20 and jacket 24 of the carrier section for accessingoptical fibers 22.

Fiber optic cable 10 also includes a web 28 connecting messenger section12 and carrier section 14. Typically, the web is formed of the samematerial that forms jackets 18,24 of the messenger and carrier sections,such as polyethylene. Preferably, web 28 comprises a plurality ofintermittent web portions such that windows 30 are defined betweenadjacent web portions as depicted in FIG. 2. Windows 30 can be useful inthat a tool can be inserted through a window 30 to sever web 28. Theaxial length L of a web portion 28 relative to an axial length W (FIG.2) of window 30 can vary within the scope of the present invention. Inother words, from the standpoint of resistance of carrier section 14 toelongation and contraction, it is preferred that length L is greaterthan or about equal to length W, such that the L:W ratio is about 1:1 toabout 50:1. More preferably about the L:W ratio is about 1:1 to about10:1. For example, window 30 can define about a 5 cm longitudinal gapabout every half meter. The L:W ratio is selected so that carriersection 14 will be effectively restrained, through the webs, againstelongation and contraction by messenger section 12. However, for lessconstraint on the messenger section, L is preferably less than W, e.g.,the ratio is about 0.25:1 to about 0.95:1.

If desired, carrier section 14 can be made longer than messenger section12 by some amount, such as about 0.2 percent. In this way, the carriersection can be substantially protected from the relatively high tensileforces to which fiber optic cable 10 may be subjected. Thus, in additionto excess fiber length (EFL) discussed hereinbelow, tensile forcesacting on the cable can be controlled, thereby controlling opticalattenuation. However, optical fibers 22 preferably have excess fiberlength, and the excess fiber length can provide protection against fiberstrain and optical attenuation.

In a preferred embodiment of the present invention, carrier section 14does not include strength members and the optical fibers are set with ahigh EFL. The greater EFL accommodates elongation of carrier section 14without transmission of stress to optical fibers 22, thereby renderingthe strength members unnecessary. In addition, the preferably generallycylindrical internal surface of a tube or jacket curves the opticalfibers creating EFL, for example, the fibers are guiding by the internalsurface in a helical path. The diameter of a preferred helical pathalong which a given optical fiber 22 extends is increased by increasingthe internal diameter (ID) of the tube or jacket surface. EFL ispreferably measured by measuring the fiber and tube lengths, subtractingthe tube length from the fiber length, and then dividing by the tubelength. EFL is then expressed as a percentage.

The preferred EFL range, for a range of tube or jacket ID sizes, isabout 0.0% to about 3.2%. More specifically, for ID sizes of about 3.0mm to about 13.0 mm, preferred EFL ranges are about 0.6% to about 3.2%,more preferably about 0.6% to about 1.5%, and most preferably about0.80% to about 1.0%. Depending on the number of fibers and the desiredhelix, the EFL ranges can vary according to the present invention. Inother words, for example, the preferred EFL for a 12 fiber cable withIDs between 3.0 mm to 13.0 mm is between about 0.5% to about 3.2%. For a72 fiber cable with tube IDs between about 3.0 mm to about 13.0 mm theEFL is preferably about 0.05% to about 2.5%. For a 144 fiber cable theEFL is preferably about 0.065% to about 2.1%, the tube ID range is about5.0 to about 13.0 mm. However, it is contemplated that tube ID sizes canrange up to about 18.0 mm or more.

Fiber optic cable 10 can include optical fibers 22 of different lengths.For instance, FIG. 2 illustrates one bundle of optical fibers 22 a thatterminate at a location spaced from the end of cable 10. It should beunderstood that FIG. 2 is schematic and not to scale. A plurality ofprogressively shorter lengths of optical fibers 22 can be employed sothat, for instance, at regularly spaced locations along cable 10 thereis at least one optical fiber 22 that terminates at each such location.

Fiber optic cable 10 of the present invention can be fabricated invarious manners. In order to form carrier section 14, core tube 20,including optical fibers 22, ripcords 26, and any water-blockingelements that may be incorporated in carrier section 14, can be drawnfrom respective pay-offs. If desired, a water-blocking tape and/or acorrugated tape armor (not shown) can then be longitudinally wrappedabout core tube 20. While carrier section 14 is being formed, at leastone and, more preferably, a plurality of strength members 16 are drawnfrom respective pay-offs and stranded together in preparation forforming messenger section 12. The internal components of messengersection 12 and of carrier section 14 are then passed through an extruderthat extrudes jackets 18, 24 that surround strength members 16 ofmessenger section 12 and core tube 20 of carrier section 14. Inaddition, the extruder forms web 28 that connect the messenger andcarrier sections. Preferably, windows 30 are formed in web 28 such as bya plunger associated with the extruder, to define a plurality ofintermittent web portions.

If it is desired to form carrier section 14 to have an overlength as awhole relative to messenger section 12, then messenger section 12 isgenerally placed under more tension than carrier section 14 during themanufacturing process. As described in detail in U.S. patent applicationSer. No. 09/102,392, which is hereby incorporated herein by reference.

A suitable EFL manufacturing process is disclosed in U.S. Pat. No.4,921,413, the disclosure of which is incorporated herein by reference.

Although fiber optic cable 10 includes core tube 20 containing theoptical fibers, the invention is also applicable to cables 10′ that donot include a core tube (FIG. 1a) such that the optical fibers aredisposed in the passageway defined by the outer jacket 24 and arecapable of contacting the inner surface of the jacket. The passageway ofthe jacket can include water-swellable elements and/or filling compounds27 if desired. In this embodiment, the ripcords are preferably locatedin the jacket wall.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

What is claimed is:
 1. A fiber optic cable, comprising: a messengersection, said messenger section comprising at least one strength memberand a jacket surrounding the at least one strength member; a carriersection being a tubeless configuration that excludes a strength member,the carrier section including at least one optical fiber and at leastone water-swellable element disposed within a passageway of a jacket ofthe carrier section; and a web connecting said respective jackets of themessenger section and the carrier section, the web comprising aplurality of web portions spaced apart lengthwise along the fiber opticcable such that windows are defined in the web, a length L of the webportions being greater than a length W of the windows.
 2. The fiberoptic cable according to claim 1, the at least one optical fiber havingan excess fiber length (EFL) greater than about 0.0 and less than about3.2%.
 3. The fiber optic cable according to claim 1, the at least oneoptical fiber being a portion of an optical fiber ribbon.
 4. The fiberoptic cable according: to claim 1, the at least one optical fiber beinga portion selected from the group consisting of bundled optical fibersand loose optical fibers.
 5. The fiber optic cable according to claim 1,the fiber optic cable being dielectric.
 6. A fiber optic cable,comprising: a messenger section, said messenger section comprising atleast one strength member and a jacket surrounding the at least onestrength member; a carrier section being a tubeless configuration thatexcludes a strength member, the carrier section including at least oneoptical fiber and a thixotropic filling compound disposed within apassageway of the jacket; and a web connecting said respective jacketsof the messenger section and carrier section.
 7. The fiber optic cableaccording to claim 6, the at least one optical fiber having an excessfiber length (EFL) greater than about 0.0 and less than about 3.2%. 8.The fiber optic cable according to claim 6, the at least one opticalfiber being a portion of an optical fiber ribbon.
 9. The fiber opticcable according to claim 6, the at least one optical fiber being aportion selected from the group consisting of bundled optical fibers andloose optical fibers.
 10. The fiber optic cable according to claim 6,the fiber optic cable being dielectric.
 11. The fiber optic cableaccording to claim 6, the web comprising a plurality of web portionsspaced apart lengthwise along the fiber optic cable such that windowsare defined in the web.
 12. The fiber optic cable according to claim 11,a length L of the web portions being greater than a length W of thewindows.
 13. A fiber optic cable, comprising: a messenger section, saidmessenger section comprising at least one strength member and a jacketsurrounding the at least one strength member; a carrier sectionexcluding a strength member, the carrier section including at least oneoptical fiber disposed within a passageway of a carrier jacket, the atleast one optical fiber having an excess fiber length (EFL) greater thanabout 0.6 and less than about 3.2%; and a web connecting said respectivejackets of the messenger section and carrier section.
 14. The fiberoptic cable according to claim 13, the carrier section further includingat least one water-swellable element.
 15. The fiber optic cableaccording to claim 13, the carrier section being a tubelessconfiguration.
 16. The fiber optic cable according to claim 13, thecarrier section further comprising a tube, the tube having the at leastone optical fiber disposed therein.
 17. The fiber optic cable accordingto claim 16, the tube having an armor tape disposed therearound.
 18. Thefiber optic cable according to claim 13, the at least one optical fiberbeing a portion of an optical fiber ribbon.
 19. The fiber optic cableaccording to claim 13, the at least one optical fiber being a portionselected from the group consisting of bundled optical fibers and looseoptical fibers.
 20. The fiber optic cable according to claim 13, thefiber optic cable being dielectric.
 21. The fiber optic cable accordingto claim 13, the web comprising a plurality of web portions spaced apartlengthwise along the fiber optic cable such that windows are defined inthe web.
 22. A fiber optic cable, comprising: a messenger section, saidmessenger section comprising at least one strength member and a jacketsurrounding the at least one strength member; a carrier sectionexcluding a strength member, the carrier section including at least oneoptical fiber disposed within a tube, an armor tape disposed about thetube, a water-swellable element disposed about the armor tape, and ajacket surrounding the armor tape, wherein the at least one opticalfiber has an excess fiber length (EFL) greater than about 0.0 and lessthan about 3.2%; and a web connecting said respective jackets of themessenger section and the carrier section.
 23. The fiber optic cableaccording to claim 22, the at least one optical fiber being a portion ofan optical fiber ribbon.
 24. The fiber optic cable according to claim22, the at least one optical fiber being a portion selected from thegroup consisting of bundled optical fibers and loose optical fibers. 25.The fiber optic cable according to claim 22, the fiber optic cable beingdielectric.
 26. The fiber optic cable according to claim 22, the webcomprising a plurality of web portions spaced apart lengthwise along thefiber optic cable such that windows are defined in the web.